|
Turbulence affects suspended particles in many ways [1], i.e. preferential concentration, dispersion, and collisions. In many particulate systems, e.g. colloids and micrometer sized particles, the particles upon collision stick together and form aggregates. The formation and evolution of these aggregates is affected by turbulence in multiple ways: On the one hand, turbulence enhances the growth of aggregates due to particle collisions; while on the other hand, it also can cause their breakup due to hydrodynamic stress. Breakup is an important process in many engineering applications and natural processes. In particular, it is one of the two main mechanisms that can stop aggregate growth in an infinite suspension and it crucially affects the particle dispersion in engineering applications. In the past the role of turbulence on aggregate breakup was mainly addressed experimentally, e.g. by measuring the evolution of the aggregate size, or by means of population balance modelling. Recently, we used direct numerical simulations of particle laden flows to explore the dynamics of breakup of small aggregates in homogeneous and wall-bounded turbulent flows [2,3]. In these simulations, the aggregates had negligible inertia and were assumed small with respect to the Kolmogorov length scale. Furthermore, breakup was assumed to occur whenever the local hydrodynamic stress, taken proportional to local energy dissipation, exceeds a predefined threshold which is a characteristic of a given type of aggregates. From measuring the evolution of dissipation along an aggregate trajectory we then obtained the breakup rate, i.e. the number of breakup events per unit time as function of the threshold dissipation, which presents an elementary quantity for describing dynamics of aggregating particles. In this work, we extend the analysis to aggregates with finite inertia. Specifically we consider the case of small and heavy aggregates which typically applies to aerosol particles. The hydrodynamic stress is taken as the sum of the shear stress and the drag stress caused by the velocity difference between the aggregate and the fluid flow, and breakup is assumed to occur once the hydrodynamic stress exceeds a predefined threshold value representing aggregate strength. We found that despite the additional stress contribution the measured breakup rate shows some remarkable similarities to the breakup rate of tracer-like aggregates in homogeneous turbulence [2]. In particular, for weak aggregates the breakup rate as a function of the critical threshold dissipation describes a power law consistent among the studies cases. Strong aggregates, on the other hand, are broken by the strong turbulent fluctuations which effect the aggregates differently. [1] Toschi and Bodenschatz, Ann. Rev. Fluid Mech.41, 375-404 (2009) [2] Babler, Biferale, and Lanotte, Phys. Rev. E 85 025301 (2012) [3] Babler et al, J. Fluid Mech. in press (2014) |